Since flexible polyurethane foams have an excellent cushioning property, they are widely used for the purpose of cushion materials such as seat cushions for vehicles. In particular, since cushion materials having a high resilience can achieve an ideal body pressure dispersion and it is very comfortable, the needs are extremely high. Also, in the seat cushions, there are simultaneously required a moderate hardness that is not too soft and too hard, as well as an excellent durability that there is a little change of the elasticity, the hardness and the thickness even if it is used for a long time.
In late years, in the field of flexible polyurethane foams, in particular of seat cushions for vehicles such as automobiles, a weight saving is required. However, when a weight saving of the flexible polyurethane foam is progressed, the hardness is damaged and the form properties tend to be deteriorated. Thus, various studies regarding maintaining and improving the hardness have been carried out.
As a method for improving the hardness of a flexible polyurethane foam, it is generally known to increase the used amount of a polymer dispersed polyol obtained by dispersing a vinyl polymer in a polyether polyol. However, by this method, another property is deteriorated.
As another method, there is a method using a specified crosslinker. For example, Patent Document 1 discloses a technology in which a polyglycerin is added as a part of a crosslinker to improve a moldability. However, in this method, the sufficient hardness of the flexible polyurethane foam is not developed.
Also, Patent Document 2 discloses a polymerization method of glycidol using an alkali metal halide as a catalyst. However, in this method, a polyol for developing a sufficient property of the flexible polyurethane foam is not obtained.
Also, Patent Document 3 and Patent Document 4 disclose an ester of a fatty acid and a polyglycerin with a primary OH of 50 wt % or more as a composition in a high crystallized material. However, these are not used for a flexible polyurethane foam.
Also, Patent Document 5 discloses a method in which a polyglycerin is used as a crosslinker together with a specified polyol to produce a flexible polyurethane foam. However, in this method, a polyol for developing a sufficient property of the flexible polyurethane foam is not obtained.
Also, Non-Patent Document 1 discloses a cyclic oligoglycidol (C18, 6 glycidol). Also, Non-Patent Document 2 discloses a method for synthesizing a polyglycidol by an addition of glycidol to a monomer having terminal OH. However, these are not used for a flexible polyurethane foam.